1. Field of the Invention
This invention relates to an automatic slack adjuster for a vehicle drum brake of the type actuated by a key rigidly mounted on a key shaft connected to a cylinder-piston activator. The said key is acting upon a pair of brake shoes through force transmitting devices each of which comprising two telescopically displaceable elements, one being an externally threaded rod, while the other is an internally threaded sleeve receiving said rod. Each of the two sleeves of the two elements is provided with a circumferential tooth rack engaging grooves extending radially on an axial shoulder surface of a wheel journalled on the key shaft, the grooves having such shape that any straight radial movements of the tooth rack relative the wheel would cause a torque on both sleeves in the direction causing increase of the total length of the two telescopically displaceable elements.
2. Description of the Prior Art
The U.S. Pat. Nos. 3,891,068 and 4,161,999 show slack adjusters of this and similar types.
Slack adjusters of the actual type should be extremely slow acting i.e. the adjustment performed per braking cycle should be very small. Otherwise there will be a substantial risk of false adjustments performed partly because of increased slack during short time intervals caused by heating the brake drum. If the automatic adjustment is sufficiently slow almost no adjustment is carried out during several subsequent braking operations performed with hot braking drums.
However, if the automatic adjustment is set to be extremely slow it will be almost impossible to check that a correct slack will be obtained because of the very large number of braking cycles required for this purpose. Therfore, it will be necessary to rely on the accuracy of the installation and the elements of the slack adjuster.
As a rule the slack adjustment may be initiated either because of excessive movements of certain brake force transmitting elements relative a stationary part of the brake or it may be initiated due to excessive relative movements of two force transmitting elements.
In the first of these two cases it is evident that false mounting of the stationary part or a subsequent displacement of said part will cause errors in the slack. In the latter of the two cases--the group to which the present invention belongs--the achievement of the correct slack is dependent on a correct installation as any deviation from movements in the correct planes and angles will cause deviations from the desired slack. It is almost impossible to check this when mounting the slack adjuster.